We are pursuing the striking observation that ablation of hypothalamic AgRP neurons in adult, but not neonatal, mice results in severe anorexia. We discovered that the anorexia is due to sudden loss of GABA signaling by AgRP neurons to the parabrachial nucleus (PBN) by showing that the lethal anorexia can be prevented by chronic infusion of a benzodiazepine GABAA receptor agonist into the PBN, but not other nuclei. We hypothesize that balanced input to the PBN maintains normal feeding and that excessive activity of the PBN (e.g. due to loss of GABA) results in anorexia. We propose to identify the source of the excitation to the PBN, as well as the neurotransmitter(s) and receptor(s) involved using pharmacological and genetic tools. We also propose to discover a gene that is specifically expressed in the critical PBN neurons that mediate anorexia, and then target Cre recombinase to that gene, which would greatly facilitate further genetic, tracing and electrophysiological studies. Our experiments indicate that mice can adapt to loss of AgRP neurons and resume normal eating, when chronically treated with a GABAA agonist (bretazenil) a 5HT3 antagonist (ondansetron), LiCl or exposed to a high-fat diet. We will explore the hypothesis that these treatments lead to adaptations in neuronal inputs or outputs of the PBN, or plasticity within the relevant PBN neurons themselves. We anticipate that these experiments will delineate a neural circuit that is important for maintenance of normal feeding behavior. We have established powerful pharmacological and genetic techniques that will allow us to identify the critical neurotransmitters and receptors that are used by neurons within that circuit. Our ultimate goals are to understand how this circuit adapts to changing environmental conditions and identify the molecular and cellular changes involved. This research is relevant to a better understanding normal and addictive feeding behavior, neuronal plasticity, and diseases such as anorexia nervosa.